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NASA's dream of sending humans to Mars by 2035 might face a new roadblock.

For the first time, researchers found that exposure to radiation in deep space could harm astronauts' brains by speeding up the the onset of Alzheimer's disease.

The study, published Dec. 31 in the journal PLOS ONE, exposed mice to a form of highly-charged radiation comparable to what astronauts would be exposed to on a mission to Mars. Mice that had been exposed to radiation were more likely to fail experiments designed to test their abilities to recall objects or specific locations.

The concept of a "career limit" refers to how long astronauts can be blasted by cosmic rays before they risk serious health problems. NASA has previously examined the potential impact of cosmic radiation on an increased risk of cancer or cardiovascular issues, but not on mental function.

“The possibility that radiation exposure in space may give rise to health problems such as cancer has long been recognized. However, this study shows for the first time that exposure to radiation levels equivalent to a mission to Mars could produce cognitive problems and speed up changes in the brain that are associated with Alzheimer’s disease," M. Kerry O’Banion, a professor at the Rochester Medical Center said in a press release.

Since the size and speed of the radiation particles used during testing enables them to pass through solid objects, including the walls and protective shielding of a spacecraft, engineers are looking at a new challenge when it comes to protecting human travelers.

"One would have to essentially wrap a spacecraft in a six-foot block of lead or concrete,” O’Banion said.